The Republic by Plato Part 4 Page 14

the guardians when inferior, and of elevating into the rank of guardians the offspring of the lower classes, when naturally superior. The intention was, that, in the case of the citizens generally, each individual should be put to the use for which nature intended him, one to one work, and then every man would do his own business, and be one and not many; and so the whole city would be one and not many.

Yes, he said; that is not so difficult.

The regulations which we are prescribing, my good Adeimantus, are not, as might be supposed, a number of great principles, but trifles all, if care be taken, as the saying is, of the one great thing, — a thing, however, which I would rather call, not great, but sufficient for our purpose.

What may that be? he asked.